Radiation in the extreme ultraviolet (EUV) and soft x-ray (SXR) regions of the electromagnetic spectrum finds use in applications that cannot be performed using visible light, such as imaging and printing of small features, diagnosis of dense large-scale plasmas, and the study of material surfaces by interaction with inner-shell electrons, to name a few. Many of these applications (for example, high-precision measurement techniques based on interferometry) further require radiation having substantial coherence which both allows radiation waves to interfere, and allows radiation to be focused onto small areas, thereby permitting high intensities to be achieved.
Synchrotron sources, high-order harmonic sources, and soft x-ray lasers have been used for generating coherent EUV/SXR radiation. Synchrotrons produce high average powers of EUV/SXR radiation that can be filtered in space and frequency to obtain a substantial coherence. Typically, these sources are multi-user facilities requiring their own buildings and costing hundreds of millions of dollars.
Direct amplification of EUV/SXR radiation in a suitable laser medium is an attractive method for generating intense beams of such radiation from a compact device. However, in order for a laser to operate efficiently and to produce significant amounts of laser output energy, the amplified beam must reach saturation intensity, whereby the amplified radiation extracts the majority of the energy stored in the population inversion. An obstacle for the development of such saturated lasers has been the large excitation power required to produce a sufficiently large population inversion in a volume having sufficient length. Successfully demonstrated EUV/SXR laser media include plasmas sustained using a powerful source of energy; for example, by focusing a high-power infrared, visible or ultraviolet laser radiation into the lasing media. Electrical energy is used to excite such a laser. To date, most saturated EUV/SXR lasers excited by this method have required large and costly pump lasers. In all cases average power output is low.
An alternative excitation method uses an electrical discharge as the energy source, thereby creating a plasma and exciting the laser upper level. In U.S. Pat. No. 4,937,832 which issued to Jorge J. Rocca on. Jun. 26, 1990, an electrical discharge through a capillary channel was demonstrated to produce a plasma column in which large EUV/SXR laser amplification was obtained. The lasing portion of the apparatus occupied a volume of about 1 m×1 m×2.5 m. A liquid-dielectric (de-ionized water or ethylene glycol, as examples), high-voltage capacitor was charged to a chosen high voltage, and discharged through the capillary channel. A high-voltage Marx generator which included a voltage multiplication circuit having at least two series connected stages, each stage containing a capacitor and a high-voltage switch, was used. The capacitors were charged in parallel to a selected voltage using a high-voltage power supply, while the switches remained open. When the switches are simultaneously closed, a voltage that is of the order of the charging voltage times the number of stages is produced. A more compact configuration was achieved by replacing the liquid dielectric capacitor with a liquid-dielectric Blumlein transmission line. This latter device occupied a volume of about 0.4 m×0.4 m×1 m, a significant reduction in size respect to laser-excited EUV/SXR lasers. A Marx generator (typical volume of about 1 m3), which includes a de-ionizing unit for achieving and maintaining sufficiently high resistivity for the liquid dielectric is generally still required.
Significant efforts have been devoted to reducing the size of saturated soft x-ray lasers from laboratory size [See, e.g., B. J. MacGowan et al., Phys. Fluids B 4, 2326, (1992); and A. Carrillon et al., Phys. Rev. Lett. 68, 2917, (1992)], to table-top size [See, e.g., J. J. Rocca et al., Phys. Rev. Lett. 73, 2192 (1994); J. Dunn et al., Phys. Rev. Lett. 84, 4834 (2000); S. Sebban, et al., Phys. Rev. Lett. 86, 3004 (2001); S. Sebban et al., Phys. Rev. Lett. 89, 253901 (2002); K. A. Jenulewicz et al., Phys. Rev. A 68, 051802 (2003); and A. Butler et al., Phys. Rev. A 70, 023821 (2004).]. The demonstration of laser amplification in transitions of Ne-like ions in a capillary discharge plasma [See, e.g., J. J. Rocca et al., Phys. Rev. Lett. 73, 2192 (1994); and J. J. Rocca et al., Phys. Rev. Lett. 77, 1476 (1996).] has led to the development of compact, short-wavelength lasers. Table-top size Ne-like Ar lasers operating at a wavelength of 46.9 nm have been developed making using water capacitors charged to high voltage (200-700 kV) from the pulsed output of Marx generators [See, e.g., J. J. Rocca et al., Phys. Rev. Lett. 77, 1476 (1996); B. R. Benware et al., Phys. Rev. Lett. 81, 5804 (1998); C. D. Machietto et al., Optics Lett. 24, 1115 (1999); A. Ben-Kish et al., Phys. Rev. Lett. 87, 1 (2001); and A. Ritucci et al., Applied Phys. B 78, 965 (2004).].
Such lasers have been used in numerous applications, including interferometry of dense plasmas [See, e.g., J. Filevich et al., Optics Lett. 25, 356 (2000).], the measurement of optical constants [See, e.g., A. Artioukov et al., IEEE J. of Selected Topics in Quant. Elec. 5, 1495 (1999).], materials ablation [See, e.g., B. R. Benware et al., Optics Lett. 24, 1714 (1999).], the characterization of soft x-ray optics [See, e.g., M. Seminario et al., Appl. Optics 40, 5539 (2001).], excitation of color centers in crystals [See, e.g., G. Tomassetti et al., Europhys. Lett. 63, 681 (2003).], nanopatterning [See, e.g., M. G. Capeluto et al. (submitted to IEEE Transaction on Nanotechnology).], photochemistry, and high resolution imaging.
Accordingly, it is an object of the present invention to provide coherent EUV/SXR radiation from a compact, high average power laser.
Another object of the present invention is to provide coherent EUV/SXR radiation at sufficiently low applied voltages that a Marx generator is not required.
Still another object of the present invention is to provide coherent EUV/SXR radiation at high repetition rates, and high average power.
Additional objects, advantages and novel features of the invention will be set forth, in part, in the description that follows, and, in part, will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.